(1) Field of the Invention The invention relates to processes and apparatus for the batchwise production of glass by melting, refining and discharging the glass, and to a vessel used to perform the processes.
(2) Technical Considerations and Prior Art
An essential step in glass production is feeding large amounts of molten, bubble-free glass continuously to processing machines. The basic theory underlying these processes is set forth in D. R. Uhlmann et al (1983), Glass-Science and Technology, Academic Press, Inc., pages 1-44.
It is known to effect melting, refining and discharging of glass in continuous troughs so that melting, refining and processing takes place simultaneously. In these troughs a melting system is connected continuously from a mixture feed point up to a feeder outlet by means of a fixed channel system. In the region of the trough proper, i.e. where the melting, refining and cooling elements are located, it is necessary to interrupt operation of the process to change individual parts or perform repairs.
Since the individual processes merge in the trough, the residence time spectrum of the melt is very large, and flow relationships cannot be separately controlled. For this reason, large structural units with large safety margins are required. As a consequence, the throughput of molten glass per unit volume (m.sup.3) of the melt vessel is low and controlling of individual processes as well as localizing disturbances is difficult. Moreover, heat losses are considerable and can be reduced further only with great difficulty.
It is also conventional to melt glass in a laboratory in platinum crucibles. In laboratories, heat is fed to the melt from the outside over the entire surface of the melt and of the crucible. However, with this approach, the heat-transfer surface per unit volume of melt is large and the exit route for gas bubbles is small. The melting output/unit volume of melt is, therefore, very large. A large melting output has considerable economical advantages. However, such advantages become significant only in melting large quantities of glass. Transferring the laboratory method to installations with large throughput while maintaining the large melting output/unit volume of melt is impossible. This is because in large installations, only the surface of the melt, rather than that of the crucible, can be exploited for heat transfer to the melt and, inter alia, the heat-transfer surface/unit volume of melt becomes smaller while the exit route for the bubbles becomes larger. Consequently, refining time becomes longer.
Processes are employed in the prior art which include direct electric resistance heating, but here again, in continuous operation, large safety volumes are necessary in order to avoid entrainment of unmolten and unrefined glass. Heat transport to the mixture lying on the melt takes place by convection and radiation from molten glass flowing along therebelow. This flow has a large velocity spectrum, but the portion of the stream having the highest velocity must flow so slowly that it inadequately entrains the molten glass. In summary, the disadvantages of continuous operation employing direct electric heating are as follows:
(1) large residence time spectrum for individual particles resulting in different conditions for reactions, mass transfer processes, and heat exchange processes;
(2) unreliability, due to failure of the entire installation upon failure of one element;
(3) fixed capacity, and
(4) difficulty in converting to other modes of production or process operation.
In order to shorten the melting or refining operation and thus increase throughput, processes have been employed in the prior art wherein at least the refining step is performed in a rotating vessel. For example, it is known from German Patent DOS No. 2,214,157 to introduce the already molten, but not yet refined glass continuously into a rotating tank. Depending on the number of revolutions, the glass is refined within a fraction of the time period required by conventional procedures. Subsequently, the finished melt is withdrawn while the vessel is in rotation. A disadvantage of this process resides in the fact that there is no possibility for transferring the glass, with the vessel in rotation, into a stationary vessel without creation of bubbles. The exiting stream has great rotational energy, but no strength. It cannot fall freely over relatively large distances without breaking away into a discontinuous stream. If the stream is collected immediately after exiting, great shearing forces arise so that bubbles are twisted into the stream. Moreover, melting and refining in the same vessel cannot be done.
German Patent DOS No. 2,259,219 discloses a high-temperature furnace equipped with plasma or arc heating and is operable either continuously or discontinuously. The melting step is performed with rotation of the crucible furnace whereby the melt forms a liquid wall coating in the form of a rotational paraboloid on the interior surface of the furnace. The thus-molten glass is substantially free of bubbles so that no subsequent refining step is required. However, this process has the disadvantage that only small amounts of glass can be melted since energy is supplied merely through the free surface of the melt. Consequently, with these processes it is difficult to supply heat for large batches of glass.
U.S. Pat. No. 2,006,947 discloses an oil-heated or gas-heated furnace wherein melting and refining take place continuously or discontinuously in a thin mixture or in a glass layer located on the paraboloid-shaped inner face of a revolving, funnel-shaped container. The glass discharge step is executed continuously or discontinuously. This process has several disadvantages. When heating with gas or oil, the energy density in the melt, required for large outputs, cannot be achieved because energy can only be supplied by way of the free surface of the melt. The large free surface promotes vaporization which, in turn, gives rise to schlieren effects. In the described arrangement, the mixture can be introduced and melted only while the container is in rotation, because if the container is at a standstill, the mixture will flow downwardly in a limited strip covering the internal surface so rapidly that the time available will not suffice for melting or refining. Since the discharge opening is open at all times, the glass content of the container can only be varied within small limits solely by a change in the number of revolutions. Moreover, no blowing or stirring is possible with this approach. These problems all arise when discharging from the rotating vessel disclosed in this patent.